There is conventionally known, as a resonance calculation program, a cross section calculation program including a step of calculating a background cross section; and a step of acquiring a resonance integral from a resonance integral table with the background cross section used as an argument, and calculating an effective cross section (effective group constants) based on the acquired resonance integral (see, for example, Patent Literature 1). Calculation equations used in the steps are made up assuming that a fuel rod is a blackness (a black body) absorbing all neutrons. The black body means an object that absorbs all neutrons whereas a white body opposite in meaning to the black body means an object that does not absorb neutrons at all. Furthermore, the calculation equations used at the steps are made up assuming that a fuel cell is a one-dimensional equivalent cylinder.